Seattle

Physicists are falling out over a high-profile claim that the speed at which gravity propagates closely matches the speed of light.

The row began after two researchers said that they had measured the 'speed of gravity' for the first time and found that, like the speed of light, it is finite.

Sergei Kopeikin, a theoretical physicist at the University of Missouri in Columbia, and Edward Fomalont, an astronomer at the National Radio Astronomy Observatory in Charlottesville, Virginia, revealed their results on 7 January, at the American Astronomical Society's annual meeting in Seattle.

The pair measured the refractive effects of Jupiter's gravitational field on the radio waves emitted by a bright, distant galaxy. From this they were able to determine that gravity propagates at close to the speed of light.

The value is similar to that predicted by Einstein, but several theorists were quick to assert that Fomalont and Kopeikin's interpretation of their results — which was widely reported around the world last week — is flawed.

“It's complete nonsense,” says Peter van Nieuwenhuizen, a physicist at Stony Brook University in New York, who has devoted much of his career to studying gravity. Clifford Will, a physicist at Washington University in St Louis, Missouri, adds that he believes that the team's experimental set-up could theoretically yield gravity's speed of propagation. But Will's calculations suggest that the effect would be too small to detect with any existing telescope.

“The experiment is wonderful, but it has nothing to do with the speed of gravity,” says Kenneth Nordtvedt, a retired physics professor at Montana State University in Bozeman. Nordtvedt says that the team is actually seeing a gravitational analogue of the force of magnetism, created by electrons moving at close to the speed of light.

“I think Dr Nordtvedt is confused,” says Kopeikin. To get his speed-of-gravity interpretation, Kopeikin used his own simplified version of Einstein's gravity equations, which other theorists, such as Nordtvedt, have yet to fully accept. “The situation is a little upsetting for us,” says Fomalont, who took the experimental measurements — adding that he hopes the quarrelling theorists will get together soon and settle their differences.